The present invention relates to a catalyst for exhaust gas purification used for purifying exhaust gas from a diesel engine by burning particulates (solid carbon fine particles and liquid or solid high-molecular weight hydrocarbon fine particles) contained in the exhaust gas; a process for producing the catalyst for exhaust gas purification; an exhaust gas purifying filter supporting the catalyst for exhaust gas purification; and a exhaust gas purifying apparatus equipped with the exhaust gas purifying filter.
The particulates contained in exhaust gas from a diesel engine have a particle size of about 1 .mu.m or less, tend to float in the air and are liable to be introduced into a human body by respiration. Moreover, the particulates also contain carcinogens. Therefore, the discharge of the particulates from a diesel engine is under increasingly strict regulations.
For removing the particulates from the exhaust gas, there is a method of collecting the particulates by the use of an exhaust gas purifying filter composed of a heat-resistant ceramic filter, heating the exhaust gas purifying filter by a heating means such as a burner or heater to burn the particulates, thereby converting the particulates to carbon dioxide, and then discharging the carbon dioxide. As the exhaust gas purifying filter, there is used a product obtained by supporting a catalyst for exhaust gas purification consisting of a metal oxide and the like on the aforesaid ceramic filter. In this case, the collected particulates can be burned at a lower temperature by virtue of the catalytic action of the catalyst for exhaust gas purification.
If the particulates can be burned at the exhaust gas temperature by using the exhaust gas purifying filter supporting such a catalyst for exhaust gas purification, a heating means need not be provided in an exhaust gas purifying apparatus, so that the structure of the exhaust gas purifying apparatus can be simplified.
However, it is difficult at present to burn the particulates sufficiently at the exhaust gas temperature even by the use of the exhaust gas purifying filter supporting the catalyst for exhaust gas purification, and simultaneous use of the exhaust gas purifying filter and a heating means is absolutely necessary. Accordingly, there is desired the development of a catalyst for exhaust gas purification having such a high catalytic activity that the particulates can be burned at a lower temperature.
It has been known that catalysts obtained by using metal oxides of, for example, Cu and/or V have a relatively high activity as catalysts for exhaust gas purification. For example, Japanese Patent Unexamined Publication No. 58-143840 (JPA-58-143840, hereinafter abbreviated as '840 Publication) discloses a catalyst for exhaust gas purification comprising a complex metal oxide containing Cu and V. Japanese Patent Unexamined Publication No. 58-174236 (JPA 58-174236, hereinafter abbreviated as '236 Publication) discloses a catalyst for exhaust gas purification obtained by adding an alkali metal to a metal oxide of, for example, Cu, V and/or Mo.
The above-mentioned conventional catalysts for exhaust gas purification, the exhaust gas purifying filters supporting this catalyst and the exhaust gas purifying apparatus using this filter, however, involve the following problems.
1) The catalyst for exhaust gas purification disclosed in '840 Publication and an exhaust gas purifying filter supporting this catalyst do not permit burning of particulates collected by the exhaust gas purifying filter at the exhaust gas temperature because the catalytic activity of the catalyst for exhaust gas purification is not sufficiently high to permit sufficient burning of the particulates at a low temperature.
2) The catalyst for exhaust gas purification disclosed in '236 Publication cannot have a high catalytic activity because in a calcination step in the production of the catalyst for exhaust gas purification, an alkali metal salt is decomposed into a highly reactive alkali metal oxide which is scattered by heat or reacts with other metal oxides present together therewith to form a complex metal oxide having a low activity (e.g. LiVO.sub.2).
3) An exhaust gas purifying filter using the catalyst for exhaust gas purification disclosed in '236 Publication is disadvantageous in that it is subject to heat deterioration and hardly retain a high catalytic activity because the catalyst for exhaust gas purification has no high resistance to the heat of exhaust gas or the heat of combustion of particulates. Moreover, the filter is poor in toxin resistance against sulfur oxides in exhaust gas.
4) In conventional exhaust gas purifying apparatus, since exhaust gas from a diesel engine cannot be sent to an exhaust gas purifying filter while being maintained at a high temperature, the exhaust gas temperature is lowered, so that the combustion temperature of particulates cannot be high enough.
The present invention solve the above conventional problems and is intended to provide a catalyst for exhaust gas purification having a high catalytic activity for the combustion of particulates and having excellent heat resistance and toxin resistance against sulfur oxides; a process for producing a catalyst for exhaust gas purification which permits easy and sufficiently high-volume production of a catalyst for exhaust gas purification having a high catalytic activity for the combustion of particulates and having excellent heat resistance and toxin resistance against sulfur oxides; an exhaust gas purifying filter having a high exhaust gas purification rate and an excellent durability; and an exhaust gas purifying apparatus having a simple structure and excellent exhaust gas purifying properties.
5) The catalyst component(s) supported on the exhaust gas purifying filter is diffused into the filter, so that the activity of the catalyst is deteriorated.
The present invention solve this conventional problem and is intended to provide an exhaust gas purifying filter having an intermediate layer for suppressing the diffusion of catalyst component(s) into the filter.
6) The catalyst supported on the exhaust gas purifying filter has an insufficient number of contact points with the components of exhaust gas to be removed, so that these components cannot be sufficiently burned and removed.
The present invention solve this conventional problem and is intended to increase the contact points of a catalyst with the components of exhaust gas to be removed, by previously treating a filter with an acid or providing a filter with a granular, heat-resistant and inorganic material, and thereby increasing the combustion rate of these components.
7) Neither conventional oxides of Cu, V or Mo, nor complex metal oxides containing Cu, V and/or Mo can have a sufficient activity.
The present invention solve this conventional problem and is intended to improve catalytic activity for the combustion of particulates in exhaust gas by controlling valences in a metal oxide obtained by replacing a part of Cu, V and/or Mo of a metal oxide comprising Cu, V and/or Mo by an alkali metal, an alkaline earth metal or a transition metal.
8) The present invention improves the activity of a conventional catalyst comprising a metal oxide and one or more sulfates and is intended to increase the contact points of a catalyst with particulates in exhaust gas by lowering the melting point by adding cesium sulfate and potassium sulfate, and thereby improve the catalytic activity for the combustion of particulates in exhaust gas.